It is known from the art that liquid mixtures can be separated by pervaporation. Pervaporation is a separation process in which a membrane in contact with a feed stream of a liquid mixture selectively absorbs one or more of the species from the feed stream. The sorbed species permeate across the membrane under the influence of a vapour pressure gradient that is produced by removing the permeate from the product side of the membrane using a vacuum or sweep gas. Industrially, vacuum is economically produced by condensing the permeate stream. This spontaneously generates a partial vacuum that drives the process. In all cases, permeate vapor is eventually condensed and the resulting distillate stream recovered as a liquid. An advantage of pervaporation over distillation is that, because of the selectivity of the membrane, pervaporation can separate azeotropic mixtures beyond the azeotropic ratio.
WO-A-2008/054 207 describes a method for the purification of a liquid by direct contact membrane distillation, in particular for the production of desalinated water from seawater or brackish water. The method comprises passing a heated vaporizing stream of a liquid over a porous membrane. The porous membrane is hydrophobic and the pores are filled with air. Thus, liquid water is unable to pass through the hydrophobic membrane, while water vapor and other gas molecules can pass through the membrane. Vapor of the liquid flows through the membrane and is condensed on the other side at a condenser surface, which surface forms a non-porous separation between a feed stream of the liquid to be purified and the (condensed) vapor. The method described in WO-A-2008/054 207 is energy efficient in that both the costs and the energy consumption of the membrane distillation system are low.
Disadvantage of this method is that this process cannot be used for separating liquid mixtures comprising hydrophobic liquids and hydrophilic liquids. When a hydrophobic/hydrophilic mixture would be applied according to the method described in WO-A-2008/054207, pore wetting would occur which may result in the membrane becoming impermeable or leakage. Liquid mixtures may in principle be separated using the method described in WO-A-2008/054207, but only with limited efficiency because the membrane is not selective. Passing the azeotrope of a mixture is impossible using this method.